Ink jet printers generally function in one of two modes: continuous stream or drop-on-demand. Ultrasonic printheads have been described in detail in a number of commonly-owned U.S. Pat. No. 4,719,476, whose contents are herein incorporated by reference. This patent in particular describes at length the creation of capillary surface waves which are generated by various means, preferably acoustically, to create standing capillary surface waves in liquid ink filled reservoirs for ejecting droplets from selected crests of the capillary surface waves on command. As one possibility described in this patent, the addressing mechanism, meaning the selection of the sites from which droplets are to be ejected, is accomplished by locally altering the surface properties of selected crests at those sites. For example, the local surface pressure acting on the selected crests or the local surface tension of the liquid within the selected crests may be changed in order to cause droplets to be ejected in a controlled manner from the selected crests.
In another commonly-owned patent, No. 4,746,929, whose contents are also incorporated herein by reference, a so-called traveling wave droplet generator (TWDG) has been described. The TWDG uses a tube that preferably extends the full width of the page on which the printing is to take place. The tube is provided with a series of apertures in a sidewall that are spaced apart from one another, and the core of the tube is filled with the liquid ink. A piezoelectric rod is mounted at on end of the core and excites traveling acoustic waves which traverse the length of the liquid column within the tube and then impinge on an absorbing element mounted at the opposite end which serves as a matching element to eliminate any reflected waves. The acoustic pressure from this traveling wave is sufficient to eject droplets in a continuous stream from each orifice in the sidewall of the tubing. The drops are ejected continuously at the pressure peak of the wave. In order to control which of the ejected drops actually impinge on the paper and leave the desired ink mark, a deflector is arranged above each orifice such that the continuously ejected ink droplets can be deflected on to the paper or into a gutter where it returns to an ink reservoir. Thus, the addressing which corresponds to the places where ink is to be deposited is determined by the electrical signals applied to the deflectors.